An evolving threat

Epidemiologists at the University of Stellenbosch, near Cape Town, South Africa, have assembled a vast collection of tuberculosis strains. With it, they're revealing how drug-resistant strains evolve and spread through human populations. Charlie Schmidt reports. The customers who patronized a small convenience store in the suburbs of Cape Town, South Africa, in 2005 didn't know of the risk they faced by walking through the door. A few would have chatted with the store's owner behind the cash register and waved goodbye to her as they left with their snacks and soft drinks.